Week Number Calculator
Find which week of the year a date falls in under ISO 8601, or enter a year and week number to see that week’s Monday-to-Sunday dates. Every week of the year is listed in a table.
- Day of the weekMonday (day 1)
- Week runsMonday 02.11.2026 – Sunday 08.11.2026
- ISO year2026
- ISO notation2026-W45-1
- Weeks in the year53
- Weeks left in the year8
ISO weeks of 2026
How it is calculated
Under ISO 8601, weeks start on Monday and week 1 is the week with the year’s first Thursday (the week holding 4 January). So a few days in early January can belong to the previous year’s last week, and a few late-December days to the next year’s week 1. Years starting on a Thursday, and leap years starting on a Wednesday, have 53 weeks; the rest have 52. Other numberings that start the week on Sunday can give a different week.
Week = ⌊(Thursday − 1 January) ÷ 7⌋ + 1; Thursday = Date − Day of the week + 4Day of the week: Monday 1 … Sunday 7. Thursday: the Thursday of the date’s week; the week belongs to the year that Thursday falls in.- Find the date’s day of the week (Monday = 1).
- Move to the Thursday of that week.
- Take that Thursday’s day of the year, divide by 7, round down and add 1.
- The week’s Monday is (day − 1) days earlier; its Sunday is 6 days after the Monday.
Example
9 July 2026 is a Thursday, day 190 of the year. ⌊189 ÷ 7⌋ + 1 = 28: the date is in week 28, which runs 6–12 July 2026. 1 January 2021 is a Friday; its Thursday is 31 December 2020, so it is in week 53 of 2020.
Frequently asked questions
What week of the year is it today?
The date field opens on today, so the result is today’s ISO week number. Pick any other date to see its week.
Why do some years have 53 weeks?
365 days are 52 weeks and 1 day (2 in a leap year). When those extra days include a Thursday, the year gets a 53rd week: every year starting on a Thursday, and leap years starting on a Wednesday. 2020 and 2026 are such years.
Why is 1 January not always in week 1?
Week 1 is the week with the year’s first Thursday. If 1 January is a Friday, Saturday or Sunday, it belongs to the previous year’s last week (52 or 53).